Week 4 RF-How do we perceive colour? Flashcards
What is the physics of light?
Isaac Newton: decomposition of light into separate wavelength components (showed that refracted light could be concentrated back into white light by having light pass through 2 glass prisms)
-Before Newton, there were beliefs that light was white and that refracted light became contaminated when it passed through something such as glass
-Netwon had a small ray of light go through a hole in wood to pass through a glass prism onto a sheet of white paper (this is how he found out about light having different colours)
-Colour is associated with electromagnetic radiation of a certain range of wavelengths
What does visible light correspond to?
Visible light corresponds to a small range of the electromagetic spectrum roughly from 400 nm (which appears blue) to 700 nm
(appears red) in wavelength.
How dependent are we on colour?
Colours can help an object stand out and make it easier to recognise and categorise it e.g., what fruit it is and whether its ripe
What type of Colour deficiencies and Achromatopsia is there?
Two kind of deficits:
-Retinal (one or more of the three kinds of cones is missing)
-Cerebral damage
- Protanopia: L missing -> you can’t distinguish red/green
- Deuteranopia: M missing -> you can’t distinguish red/green
- Tritanopia: S missing -> you can’t distinguish blue/yellow
- About 5-10% of individuals have colour deficiencies
- Mostly males because of the link with the Y chromosome. Deuteranopia is the most common case. (Dogs are dichromats with only two cones peaking at 429 and 555 nm
and 555 nm).
Achromatopsia:
1. Cerebral: the damage is to the cortex, more specifically area V4, in the inferior part of the occipital lobe, in the lingual and fusiform gyri (Zeki, 1973).
- Cerebral Achromatopsia: difficulty in perceiving colours, world is in shades of grey
- Cerebral Hemiachromatopsia: relatively normal vision in one half of the visual field, but sees the other half in shades of grey
What is the sensitivity for wavelengths for the 3 different type of cone cells and rod cells?
-Small = 437nm
-Rods = 498nm
-Medium = 533nm
-Large = 564nm
What are the chromatic pathways?
There are 3 channels:
1. An achromatic non-colour channel which combines the activity of M-L wavelength cones. Contains non-spectrally opponent input with colours not seen in visible light e.g., brown.
- Blue-yellow channel which represents the difference between the sum of M and L on one hand and short wavelength cones on the other.
- Red-green channel which represents the difference between the activity levels in the M and L wavelength cones and the difference in where that information is coming from.
What is the Parvocellular stream from the eye to the cortex?
Retinal origin: P cells
Location: Fovea
Colour: √ -
Sensitivity to contrast: Low
Spatial resolution: High
Temporal resolution: Low (sustained)
Sensitive to: Shape & Colour
What is the Magnocellular stream from the eye to the cortex?
Retinal origin: M cells
Location: Evenly
Colour: -
Sensitivity to contrast High
Spatial resolution Low
Temporal resolution: High (transient)
Sensitive to: Motion & Depth
What are the Opponent Colours?
-Red/Green and Blue/Yellow (plus Black/White)
-The idea was first proposed by Ewald Hering in 1878.
-We are not sure about the reason for this hard-wiring, but there are some suggestions
from information theory about matching internal dimensions to the main factors that
describe light on our planet e.g., why the sun shines and down on us.
-So, if we see a red object, it is not because the longwave-sensitive (‘red’) cone sends a message, it is because a nerve cell that COMPARES the simulation of the red and green cones send a message saying: “more
longwave than mediumwave light”.
Define Hue
Hue is what we could loosely refer to as colour, as in red, yellow, blue etc. Hueless or achromatic colours include black, gray, white.
Define Saturation
Saturation refers to purity, or how much grey there is in a colour. Pink for instance differs from Red mainly because of saturation. Brown also tends to be unsaturated.
Define Brightness
Brightness goes from complete dark to dazzling, self-luminous white.
- These are three dimensions (i.e., hue, saturation, brightness) derived from
people’s judgements of colours. They are the three dimensions of the human colour space.
What is Colour Contrast?
-Colour contrast occurs when we perceive colour so the target colour shifts away from the surrounding colours (inducers)
-Simultaneous colour contrast=they look life different colours but they’re actually the same just surrounded by different colours.
What is Colour Assimilation?
The colour appearance of the surface dependent on the colours surrounding it.
What is meant by the Colour after effects?
-The system is clever and looks at contrasts not just on the basis of what is next to what. Compare the simultaneous brightness effect with the White effect.
